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DNA Methylation, presented by Steve Jacobsen.
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Cytosine DNA methylation at the
fifth position of the cytosine ring
is the most prevalent
modification of DNA
found in eukaryotic organisms.
And DNA methylation
is a major determinant
of so-called epigenetic
gene regulation.
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Epigenetics is defined
as heritable changes
in gene expression
which are not associated
with changes in the
sequence of the DNA.
Instead, epigenetic
inheritance is based
on stable, alternative chromatin
architectures at specific loci.
DNA methylation plays a major
role in epigenetic inheritance,
because DNA methylation patterns
can be stably inherited from cell
to cell during mitosis, and
in some cases from generation
to generation during meiosis.
While there are a few
exceptions to this general rule,
DNA methylation is usually
associated with gene silencing.
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DNA methylation is a
very ancient phenomenon
and in fact evolved
first in bacteria
as restriction
modification systems that
serve as a defense against
foreign DNA, such as viruses.
These methylation
restriction systems
are composed of specific DNA
methyltransferase enzymes that
act at short, palindromic sequences
and restriction enzymes that cleave
this same sequence only
if it is unmethylated.
In this way, bacteria
can distinguish
self DNA from non-self DNA.
And foreign, incoming,
unmethylated DNA
can thus be recognized
and destroyed.
In eukaryotic organisms,
DNA methylation
has evolved into a mechanism
that allows dividing cells
to stably inherit
states of gene activity.
DNA methylation is
involved in a great number
of epigenetic regulatory
processes found throughout all
of the major eukaryotic
groups, including
fungi, plants, and animals.
